Detection of specific target analytes, or chemical compounds, is important for many applications, including for example, detecting whether the concentration of analytes exceeds flammability limits. Target analytes are detected by sensors operating according to different detection mechanisms, known in the art. Most sensors employ a sensing component that is physically modified in the presence of specific analytes present in the environment. Thus, a sensor typically comprises a probe that includes both the sensing component and a probe body housing (including terminals for transmitting an output). The terminals are typically coupled to a processor, also part of the sensor, which analyzes the outputs received from the sensor probe. Such processor is coupled to a user interface, typically containing an indicating device, which signals when concentration of an analyte has exceeded threshold values.
Many sensors employ a sensing component that is a sensor film. Many sensor films swell, increasing in volume, while in the presence of the analytes. Various sensors available in the art utilize the physical changes in the sensor film to determine concentration of analyte present. Such sensors may include optical sensors, such as fiber optic sensors, where a beam of light is projected through an optical fiber at a sensor film cladding, and physical changes (e.g. refractive index or color) in the film are monitored. Such changes in refractive index occur when analytes are absorbed and change the physical properties of the cladding (including volumetric changes). Other sensors include sound acoustic wave sensors (SAWS), which project ultrasonic waves through the sensor film between transducers, and likewise detect any modifications in the properties of the sensor film (primarily the mass), translating those changes to the concentration of analyte present.
Another type of sensor film is a conductiometric sensor, more particularly, a polymer-absorption chemiresistor sensor. A polymer-absorption chemiresistor has a polymer film sensor exposed to a surrounding atmosphere containing target analytes (chemical compounds). An electrical charge is applied across the polymer film. The polymer absorbs target analytes and this results in a volumetric change of the film, and hence the electrical resistance of the film.
While current chemiresistor sensors perform adequately for their intended uses, they are subject to improvement. Specifically, the detection response of the sensor is gradual. The electrical resistance of the sensor gradually increases once the sensor film has been exposed to the analyte. This gradual increase may require a long period of time before reaching a threshold value beyond which a decision is made to turn off the machine supplying the analyte.
In one prior art detection system, the electrical resistance of a sensor gradually increases after the sensor has been exposed to the analyte. FIG. 1, illustrates an exemplary graph of a typical detection response (R vs. time) of one prior art detection system. Prior systems generally measure the electrical resistance of the sensor over a period time, which requires a long period of time before a user using the sensor is informed that the sensor has reached a threshold value Rth. As shown in this example, the threshold value of the sensor, if selected to be twice its nominal value, would result in a response time of >1400 seconds.
The detection of the flow rate of water is also important in many applications, including for example, detecting whether the amount of water being dispensed in a refrigerator icemaker exceeds overflowing limits. The flow rate of water is detected by sensors operating according to different detection mechanisms, known in the art, such as thermo-anemometers. Traditional thermo-anemometers typically include an anemometer temperature sensor disposed in a stream of water to measure the downstream temperature and a another anemometer temperature sensor disposed in the stream of water to measure the upstream temperature. The anemometer temperature sensors can be internally heated thermistors, externally heated thermistors, or other types of temperature sensors.
The anemometer sensor measuring the upstream temperature compensates for any fluctuations in water temperatures that might bias the reading of the anemometer sensor measuring the downstream temperature. The thermo-anemometer subtracts the upstream temperature from the downstream temperature to determine flow rate. By using various equations and thermal sensing principles, such as the Seebeck Effect, the temperature result is then correlated to a flow rate. Other methods can be used to measure the flow rate of water, such as, measuring the heat loss of a heat source (heat source heating the thermistors) that is exposed to the flow of the fluid and using the appropriate equations and principles to correlate the temperature measurement to a flow rate.
In an application using the prior art technology, such as the refrigerator icemaker example, the amount of water dispensed depends on line pressure, which determines the flow rate of water once the valve is opened. If the flow of water is timed, then the volume dispensed into the ice tray can vary significantly. By measuring the flow rate just after opening the icemaker dispense valve, it is possible to more accurately fill the ice tray to an appropriate level each time. However, at high flow rates of more than 0.75 GPM, the valve must only be open for a short period of time, and the measurement of flow must take place within less than 2 seconds after the valve opens. This is achieved by measuring the temperature (T) of the heat source of the thermo-anemometer over time (t), as done in the prior art. Since it takes a long period of time for the temperature (T) of the heat source to reach a steady state value, traditional thermo-anemometers are often inadequate for these applications. FIG. 2, illustrates an exemplary graph of a typical response (Temperature vs. time) of the prior art system. When temperature (T) is plotted versus time (t), the steady state value for T is not reached until 1.5 to 2.0 seconds after the water valve opens. In this example, the temperature was measured both at a flow rate of 0.15 GPM and 0.75 GPM.
There is a need for a signal conditioning technique for improving the response time of a sensing device, such as a chemiresistor sensor and a water flow sensor, thus improving the reaction response time for the user of the sensor.